Prior art data communications systems use a variety of methods for coding data into an analog medium. Such well known methods include amplitude modulation, frequency modulation and phase modulation. Two forms of phase modulation are commonly used: BPSK (Binary Phase Shift Keyed) and QPSK (Quadrature Phase Shift Keyed). In a BPSK system, there are two phases: an in-phase signal and a 180.degree. out of phase signal. During each baud (i.e. digital symbol transmission cycle), the transmitter sends one of the two signals. The phase sent determines the value of the bit transmitted (zero or one). A single binary bit per baud is conveyed from transmitter to receiver during each baud time in a BPSK system.
In a QPSK system, there are four phases: an in-phase signal, a 180.degree. out of phase signal, a +90.degree. phase signal, and a -90.degree. phase signal. During each baud, the transmitter sends one of four baud signals. The phase sent determines the value of each of two transmitter bits. Two binary bits per baud are thus conveyed from transmitter to receiver during each baud time.
Both BPSK and QPSK systems have a distinct set of characteristics that make one form of coding better in some circumstances while the other method is better for other situations. QPSK has the advantage of getting more data into less bandwidth. At the same time, separation between phases in a QPSK system is half that of BPSK phases (phases are only 90 degrees apart in QPSK instead of 180 degrees for BPSK). Thus, some noise immunity is lost in QPSK. If there is any kind of impairment on a communication channel which could modulate the phase of the carrier, BPSK would be more reliable than QPSK. Therefore, QPSK is more efficient in getting more data into a limited bandwidth while BPSK is more robust.
BPSK and QPSK phase modulation techniques may be employed in a variety of adverse communication environments. One such adverse environment is a power distribution network. Although a power line may be used as a data communication medium, the level of noise present on the power line makes effective communication very difficult. A typical AC power line network is used for power distribution to a number of electric devices connected thereto. Such devices cover a wide range of applications including hair dryers, television sets, computers and specialized factory tooling. Each type of device conducts a significant level of noise back onto the power lines. Different devices produce a different type and degree of noise. This diverse power line noise problem severely impairs the efficient and reliable operation of any power line communication system.
Another problem hindering any power line communication system is signal attenuation. Due in part to the diverse impedance levels of the electric devices being used with a power line network, transmitted communication signals may suffer greater than 40 dB attenuation before being captured by a receiver. The significant signal attenuation problem in combination with the noise problem renders effective signal line communication difficult.
Time varying conditions on a typical AC power line have been observed to cause sudden, periodic shifts in the carrier phase of received data communication signals. Phase shifts up to 45 degrees in the carrier frequency range of 50 KHz to 150 KHz are relatively common. In the carrier frequency range of 100 KHz to 150 KHz, a phase modulation of 22 degrees is relatively common, while 45 degree shifts occur only when certain electrical devices are powered on. These phase shifts severely impair the capability of any QPSK receiver during the time the interference is present. At other times, for example when certain electrical devices which produce large phase variation are powered off, power line interference is reduced to a level at which QPSK operation is not a reliability risk. It will be apparent to those skilled in the art that other adverse data communication environments other than power line distribution networks present impairments to the faster QPSK method.
It is therefore desirable to support both BPSK (for robustness) and QPSK (for speed) modes in a single data communication system. The QPSK mode can be enabled when low levels of interference allow fast transmission; BPSK mode may be enabled when greater reliability is necessary in a noisy environment. It is also desirable to provide a means for switching between BPSK and QPSK modes that is tolerant of bit errors occurring during transmission of the mode information.